Incarceration plays a central role both within criminology and in society at large. In terms of crime control, its purpose is to prevent offenses through deterrence, incapacitation, and rehabilitation. Because incarceration is partly intended to serve these instrumental purposes—and given that it is a costly institution, both economically and in human terms—evaluating its effectiveness is essential. In addition, incarceration has become increasingly prominent in political discourse, with reforms leading to rising incarceration rates in many countries, including Sweden. Despite this, robust evidence on the impact of incarceration remains ambiguous—both in terms of its overall effect on prison-sentenced individuals and the impact of specific interventions during imprisonment.

This thesis addresses both historically central components of incarceration and more contemporary issues that have emerged. These include the length of a prison stay and its deterrent effect on post-sentence reoffending (Article I), the number of crimes prevented due to the incapacitation effect of incarcerating an offender (Article II), and the effect of electronic monitoring—either as an alternative to incarceration (Article III) or as an early release intervention (Article IV). The outcomes examined in these studies extend beyond recidivism and include measures of employment and mortality as well.

The population studied in this thesis consists of first-time prison-sentenced individuals or low-risk inmates. These individuals hold particular policy relevance, as they are often situated on the margin between receiving a custodial or non-custodial sanction. By shedding light on this relatively large group, the findings from this thesis help inform policy decisions about where this margin should be drawn.

Beyond the subject of incarceration, the studies in this thesis share a common methodological approach, relying on quasi-experimental or natural experiment designs to more accurately estimate the effects of interest. The rigor of this thesis also stems from its use of a wide set of linked Swedish administrative registers, which enable effects to be measured not only in the short term but also over the long term.

The findings in this thesis show that the duration of incarceration does not affect post-sentence recidivism rates. This holds true regardless of whether the prison stay is shortened or extended. These findings suggest weak support for a specific deterrent effect, calling into question policies that assume longer sentences reduce future offending. An incapacitation effect is, however, identified, although it appears modest among low- to mid-risk inmates—supporting the consideration of non-custodial alternatives.

Since non-custodial interventions either reduce the duration of incarceration or replace prison stays entirely, concerns may arise regarding potential reductions in the crime-controlling capacity that might otherwise be expected if the sentence were served in prison in its entirety. However, this thesis does not find support for such concerns. On the contrary, it shows that electronic monitoring—whether used to replace the entire prison stay or only part of it—reduces long-term recidivism rates while also increasing labor market attachment. These findings have important implications for non-custodial alternatives to imprisonment and prison reintegration programs and, more broadly, how criminal justice policies should be shaped.